Defense Secretary Dick Cheney, seeking to assert his control of the Pentagon, scolded the Air Force's top officer Friday for negotiating with Congress over plans to strengthen the nation's land-based nuclear missile force.

After one week on the job, the former Wyoming congressman showed no reluctance to issue a public rebuke to Gen. Larry D. Welch, a member of the Joint Chiefs of Staff and one of the nation's top military men.Cheney, in his first meeting with reporters since assuming his new post, said the Air Force chief of staff's talks with congressional leaders were "inappropriate for a uniformed officer."

Welch, a four-star general, met in recent days with key lawmakers to try to resolve the longstanding impasse over how the United States should modernize its land-based nuclear missiles. While the Air Force and both the Reagan and Bush administrations have supported buying new 10-warhead MX missiles and basing them on railroad cars, congressional Democrats have urged that a much smaller, single-warhead mobile missile be bought instead.

Welch suggested the possibility of removing the existing 50 MXs from silos in Wyoming - Cheney's home state - and putting them on rail cars, while moving ahead to build and deploy 300 Midgetman missiles, a Pentagon official said.

Congressional Democrats, including Rep. Les Aspin of Wisconsin, chairman of the House Armed Services Committee, support the proposal because it would halt MX procurement at 50 missiles and allow procurement of the truck-mounted Midgetman to proceed.

"Gen. Welch was freelancing - he was not speaking for the department," Cheney said sternly. "He was obviously up there on his own hook, so to speak."

Cheney said he was displeased by the ad hoc lobbying. "I'm not happy with it, frankly," he said. "I think it's inappropriate for a uniformed officer to be in a position where he's in fact negotiating an arrangement."

Cheney said any decision over how many and what type of missile to recomend buying was weeks away. "To say that a compromise is near, I think, would be premature," he said.

He added that he hoped to discuss the matter with Welch personally.

"Everybody's entitled to one mistake," he said, apparently unaware of last year's $500,000 Air Force proposal to renovate an Air Force house for Welch - a proposal Congress rejected.

Buying 50 more MX missiles and basing them on rail cars would cost $9.1 billion, while putting the existing silo-based missiles aboard rails - giving them the ability to elude a Soviet strike - would cost about $5.6 billion.